1. Field of the Invention
The present invention relates generally to a semiconductor structure and a process thereof, and more specifically to a semiconductor structure and a process thereof, that forms a metal oxide layer having a changing energy bandgap along a direction perpendicular to the surface of a substrate.
2. Description of the Prior Art
Poly-silicon is conventionally used as a gate electrode in semiconductor devices, such as metal-oxide-semiconductors (MOS). But with the trend towards scaling down the size of semiconductor devices, conventional poly-silicon gates face problems such as lower performances due to boron penetration and unavoidable depletion effect. This increases the equivalent thickness of the gate dielectric layer, reduces the gate capacitance, and worsens the driving force of the devices. Therefore, work function metals that are suitable to be used as high-K gate dielectric layers are employed to replace the conventional poly-silicon gate to be the control electrode. The high-K gate dielectric layer is formed by a single material, so that the high-K gate dielectric layer has only a single dielectric constant, meaning a single energy bandgap.